1. Field of the Invention
The present invention relates to friction clutches, in particular for motor vehicles with internal combustion engines, including a clutch casing, a pressure plate which is guided in a rotationally fixed manner with respect to the clutch casing, at least one clutch disk and a spring device comprising from about 40 to 65% nickel.
2. Background of the Invention
Internal combustion engines provide a torque which can be used for drive purposes only within the speed range between idling speed and nominal speed. Motor vehicles therefore generally require a transmission and a device for separating the engine from the drive train. In the case of manual transmissions, a friction clutch is typically used for this purpose, and this clutch also allows an initially stationary vehicle to be driven off. Friction clutches of the prior art comprise a clutch casing, in which a pressure plate is guided in a rotationally fixed manner and is acted on by spring elements, and at least one clutch disk which in the at-rest state, which in this case is the engaged state, is clamped between the pressure plate and the torque input part as a result of the spring action. The clutch disk, which is connected to the output part, e.g. the transmission shaft, is thus entrained with the pressure plate and the input part as a result of the friction and therefore executes essentially the same rotational movement as the input part and output part. The force generated by the spring devices therefore must not fall below a minimum level in operation, in order for the engine torque to be reliably transmitted. In addition, the elasticity of the spring material is important, since the friction clutch is disengaged by elastic deformation of the spring element or elements. Since the disengagement mechanism is subject to friction, it is often useful for it to be at least partly coated with low-friction, hard materials. The friction clutches of the prior art usually have spring devices made from heat-treated steel, such as for example 50CrV4. The coating of the spring devices with hard material, such as for example chromium, is known, for example, from P 35 42 847.3. However, it has emerged that the spring devices, particularly when used in high-power engines, are heated to temperatures at which these conventional spring device materials may lose their elastic properties and their strength in the high-temperature phase, and under certain circumstances this loss may even be irreversible. Such reversible or irreversible setting losses of the spring elements may reduce the pressure force which they exert. It is therefore necessary to use relatively strong spring devices which ensure high torque transmission even under high thermal loads. A disadvantage, however, is that they also increase the inertial mass.